U.S. patent application number 17/056027 was filed with the patent office on 2021-08-12 for method for producing porous electrodes for electrochemical cells.
The applicant listed for this patent is Robert Bosch GmbH. Invention is credited to Harald Bauer, Leonore Glanz, Calin Iulius Wurm.
Application Number | 20210249647 17/056027 |
Document ID | / |
Family ID | 1000005609299 |
Filed Date | 2021-08-12 |
United States Patent
Application |
20210249647 |
Kind Code |
A1 |
Wurm; Calin Iulius ; et
al. |
August 12, 2021 |
METHOD FOR PRODUCING POROUS ELECTRODES FOR ELECTROCHEMICAL
CELLS
Abstract
The invention relates to a method for producing an
electrochemical cell comprising at least one porous electrode (2'),
the method comprising at least the following method steps: (a)
providing an electrode composition in the form of a homogeneous
mixture comprising (i) at least one particulate active material
(3); (ii) at least one particulate binder (5); (iii) at least one
particulate pore-forming agent (4); and (iv) optionally at least
one conducting additive (6); (b) forming a mouldable mass from the
electrode composition; (c) applying the electrode composition to at
least one surface of a substrate (1) to obtain a compact electrode
(2); (d) producing an electrochemical cell comprising at least one
compact electrode (2) which comprises the electrode composition
according to method step (a); and (e) heating the at least one
compact electrode (2) to liquefy the at least one particulate
pore-forming agent (4); and/or (f) bringing the compact electrode
(2) into contact with at least one liquid electrolyte composition
or at least one liquid constituent of an electrolyte composition
for an electrochemical cell which is capable of at least partially
dissolving the at least one particulate pore-forming agent (4) to
obtain a porous electrode (2), wherein method steps (a), (b), (c),
(d) and (e) are carried out substantially without solvents.
Inventors: |
Wurm; Calin Iulius;
(Meitingen, DE) ; Bauer; Harald; (Ehningen,
DE) ; Glanz; Leonore; (Asperg, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Robert Bosch GmbH |
Stuttgart |
|
DE |
|
|
Family ID: |
1000005609299 |
Appl. No.: |
17/056027 |
Filed: |
May 15, 2019 |
PCT Filed: |
May 15, 2019 |
PCT NO: |
PCT/EP2019/062413 |
371 Date: |
November 17, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/0471 20130101;
H01M 4/0404 20130101; H01M 4/139 20130101; H01M 4/62 20130101; H01M
4/043 20130101; H01M 2004/021 20130101; H01M 2220/20 20130101; H01M
10/0525 20130101 |
International
Class: |
H01M 4/139 20060101
H01M004/139; H01M 10/0525 20060101 H01M010/0525; H01M 4/04 20060101
H01M004/04; H01M 4/62 20060101 H01M004/62 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2018 |
DE |
10 2018 207 773.8 |
Claims
1. A process for producing an electrochemical cell comprising at
least one porous electrode (2'), wherein the process comprises at
least the following process steps: (a) provision of an electrode
composition in the form of a homogeneous mixture comprising (i) at
least one particulate active material (3); (ii) at least one
particulate binder (5); (iii) at least one particulate pore former
(4); and (iv) optionally at least one conductive additive (6); (b)
formation of a shapeable composition from the electrode
composition; (c) application of the electrode composition to at
least one surface of a substrate (1) to give a compact electrode
(2); (d) production of an electrochemical cell comprising at least
one compact electrode (2) which comprises the electrode composition
as obtained in process step (a); and (e) heating of the at least
one compact electrode (2) in order to liquefy the at least one
particulate pore former (4); and/or (f) contacting the compact
electrode (2) with at least one liquid electrolyte composition or
at least one liquid constituent of an electrolyte composition for
an electrochemical cell, which is able to at least partially
dissolve the at least one particulate pore former (4) so as to
obtain a porous electrode (2'), where the process steps (a), (b),
(c), (d) and (e) are carried out largely without solvents.
2. The process as claimed in claim 1, wherein the electrode
composition is provided in process step (b) in the form of a
shapeable composition which is obtained by introduction of kinetic
and/or thermal energy.
3. The process as claimed in claim 1, wherein the process step (c)
comprises a step in which the electrode composition is
compressed.
4. The process as claimed in claim 1, wherein the process steps
(a), (b), (c) and (d) are carried out at a temperature at which the
particulate pore former (4) is present as solid.
5. The process as claimed in claim 1, wherein the particulate pore
former (4) is a constituent of an electrolyte composition for an
electrochemical cell.
6. The process as claimed in claim 1, wherein the liquid
constituent of an electrolyte composition is a constituent which is
liquid at room temperature of an electrolyte composition for an
electrochemical cell.
7. The process as claimed in claim 1, wherein the particulate pore
former (4) is selected from among at least one lithium salt, at
least one organic carbonate which is solid at room temperature
and/or at least one additive which can be used for improving the
properties of the electrolyte.
8. The process as claimed in claim 1, wherein the process steps (e)
and (f) are carried out simultaneously.
9. A porous electrode (2') for an electrochemical cell (1), which
is obtained by a process as claimed in claim 1.
10. (canceled)
11. A vehicle comprising an electrochemical cell (1) including a
porous electrode (2') obtained by the process as claimed in claim
1.
12. The vehicle as claimed in claim 10 wherein the vehicle is a
vehicle having a conventional internal combustion engine (ICE), an
electric vehicle (EV), a hybrid vehicle (HEV) or a plug-in hybrid
vehicle (PHEV).
13. A process for producing an electrochemical cell comprising at
least one porous electrode (2'), wherein the process comprises at
least the following process steps: (a) provision of an electrode
composition in the form of a homogeneous mixture comprising (i) at
least one particulate active material (3); (ii) at least one
particulate binder (5); (iii) at least one particulate pore former
(4); and (iv) optionally at least one conductive additive (6); (b)
formation of a shapeable composition from the electrode
composition; (c) application of the electrode composition to at
least one surface of a substrate (1) to give a compact electrode
(2); (d) production of an electrochemical cell comprising at least
one compact electrode (2) which comprises the electrode composition
as obtained in process step (a); and (e) heating of the at least
one compact electrode (2) in order to liquefy the at least one
particulate pore former (4); where the process steps (a), (b), (c),
(d) and (e) are carried out largely without solvents.
14. The process as claimed in claim 13, further comprising: (f)
contacting the compact electrode (2) with at least one liquid
electrolyte composition or at least one liquid constituent of an
electrolyte composition for an electrochemical cell, which is able
to at least partially dissolve the at least one particulate pore
former (4) so as to obtain a porous electrode (2').
15. A process for producing an electrochemical cell comprising at
least one porous electrode (2'), wherein the process comprises at
least the following process steps: (a) provision of an electrode
composition in the form of a homogeneous mixture comprising (i) at
least one particulate active material (3); (ii) at least one
particulate binder (5); (iii) at least one particulate pore former
(4); and (iv) optionally at least one conductive additive (6); (b)
formation of a shapeable composition from the electrode
composition; (c) application of the electrode composition to at
least one surface of a substrate (1) to give a compact electrode
(2); (d) production of an electrochemical cell comprising at least
one compact electrode (2) which comprises the electrode composition
as obtained in process step (a); and (f) contacting the compact
electrode (2) with at least one liquid electrolyte composition or
at least one liquid constituent of an electrolyte composition for
an electrochemical cell, which is able to at least partially
dissolve the at least one particulate pore former (4) so as to
obtain a porous electrode (2'), where the process steps (a), (b),
(c), and (d) are carried out largely without solvents.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a process for producing a porous
electrode for an electrode. The invention also relates to an
electrode and the use of an electrode in an electrochemical
cell.
[0002] Electrodes for electrochemical cells, for example electrodes
for lithium-containing battery cells or electrodes for fuel cells,
generally comprise at least one particulate material (e.g.
particulate active materials and/or conductive additives) which is
bonded by means of at least one binder to give an agglomerated
electrode composition. A very homogeneous distribution of all
constituents of the electrode composition, i.e. both all
particulate materials and all binders, is critical for the quality
of the electrodes. Two fundamentally different processes have been
described for this purpose in the prior art. The electrodes
obtained by means of these processes differ first and foremost in
terms of their porosity. A high porosity is advantageous in the
electrode to bring about rapid diffusion of the charge carriers
(e.g. lithium ions).
[0003] In the slurry process, an active material slip in which the
particulate materials are dispersed in a solution composed of a
polymeric binder and a suitable solvent is produced. The slip is
subsequently applied to a current collector in a coating process.
The solvent is then removed by drying so as to give a porous layer
of the particulate materials and the binder on the surface of the
current collector.
[0004] US 2015/0357626 A1 and US 2010/0015327 A1 describe processes
of this type in which a pore former is additionally added to the
active material slip and is dissolved out of the electrode in a
later step. The porosity of the electrode is increased further in
this way. As preferred pore former, mention is made of ethylene
carbonate.
[0005] As an alternative, a process for the solvent-free production
of an electrode composition which can be applied to the surface of
a current collector or be processed to form free-standing electrode
sheets has been described in, for example, US 2015/303481. In this
process, an electrode composition is provided in the form of a
shapeable composition comprising at least one electrode active
material and at least one polymeric binder and optionally at least
one conductive additive. Fibrils are formed from the binder
particles by introduction of shear forces (e.g. by use of
mechanical mills such as jet mills or ball mills), and these
fibrils bring about agglomeration of the electrode composition. The
shapeable composition can, for example, be shaped by means of an
extruder and/or calender to give a stable, free-standing electrode
sheet and applied to a current collector. Alternatively, the
shapeable composition can be applied directly to a current
collector. The electrode obtained displays a comparatively low
porosity which is reduced by compression processes during extrusion
or calendering. This has the consequence that the active material
is less accessible to the charge carriers. In addition, the
otherwise homogeneous distribution of the active material particles
and the binder is impaired by the calendering processes at elevated
temperatures which are frequently employed. This frequently leads
to agglomeration of polymer constituents of the binder on the
surface of the electrode, as a result of which the accessibility
for the charge carriers decreases further.
SUMMARY OF THE INVENTION
[0006] The invention provides a process for producing an
electrochemical cell comprising at least one porous electrode,
wherein the process comprises at least the following process
steps:
[0007] (a) provision of an electrode composition in the form of a
homogeneous mixture comprising [0008] (i) at least one particulate
active material; [0009] (ii) at least one particulate binder;
[0010] (iii) at least one particulate pore former; and [0011] (iv)
optionally at least one conductive additive;
[0012] (b) formation of a shapeable composition from the electrode
composition;
[0013] (c) application of the electrode composition to at least one
surface of a substrate to give a compact electrode;
[0014] (d) production of an electrochemical cell comprising at
least one compact electrode which comprises an electrode
composition as obtained in process step (a); and
[0015] (e) heating of the at least one compact electrode in order
to liquefy the at least one particulate pore former; and/or
[0016] (f) contacting the compact electrode with at least one
liquid electrolyte composition or at least one liquid constituent
of an electrolyte composition for an electrochemical cell, which is
able to at least partially dissolve the at least one particulate
pore former so as to obtain a porous electrode,
where the process steps (a), (b), (c), (d) and (e) are carried out
largely without solvents.
[0017] The electrode composition comprises at least one particulate
active material, at least one particulate binder and at least one
particulate pore former. In general, the particulate components
thus have an average particle diameter of from 1 nm to 1 mm,
preferably from 100 nm to 100 .mu.m and in particular from 0.5
.mu.m to 30 .mu.m.
[0018] As particulate active material, it is in principle possible
to use any material which is suitable as active material for
electrochemical cells. Particular preference is given to active
materials for negative electrodes and/or positive electrodes for
lithium ion batteries. These include, for example as active
material for the negative electrode of a lithium ion battery,
amorphous silicon which can form alloy compounds with lithium
atoms. However, carbon compounds such as graphite are also worthy
of mention as active material for negative electrodes. Oxidic
active materials for the negative electrode are also known to a
person skilled in the art. Mention may be made in particular of
Li.sub.4Ti.sub.5O.sub.12, TiO.sub.2, H.sub.2Ti.sub.12O.sub.25 and
mixtures thereof. As active material for the positive electrode of
a lithium ion battery, it is possible to use, for example,
lithiated intercalation compounds which are able to take up and
release lithium ions reversibly. The active material of the
positive electrode can typically comprise a composed oxide and/or
phosphate containing at least one metal selected from the group
consisting of cobalt, magnesium, nickel, and also lithium.
Preferred examples are, in particular, LiMn.sub.2O.sub.4,
LiFePO.sub.4, Li.sub.2MnO.sub.3,
Li.sub.1.17Ni.sub.0.17Co.sub.0.1Mn.sub.0.56O.sub.2, LiCoO.sub.2 and
LiNiO.sub.2. Mention may also be made of compounds of the formula
LiNi.sub.1-xM'.sub.xO.sub.2, where M' is selected from among Co,
Mn, Cr and Al and 0.ltoreq.x<1. Examples encompass
lithium-nickel-cobalt-aluminum oxides (e.g.
LiNi.sub.0.8Co.sub.0.15Al.sub.0.05O.sub.2; NCA) and
lithium-nickel-manganese-cobalt oxides (e.g.
LiNi.sub.0.8Mn.sub.0.1Co.sub.0.1O.sub.2; NCM (811) or
LiN.sub.0.33Mn.sub.0.33Co.sub.0.33O.sub.2; NCM (111)); compounds of
the formula Li.sub.1+xMn.sub.2-yM.sub.yO.sub.4 where x.ltoreq.0.8,
y<2; Li.sub.1+xCo.sub.1-yM.sub.yO.sub.2 where x.ltoreq.0.8,
y<1; Li.sub.1+xNi.sub.1-y-zCo.sub.yM.sub.zO.sub.4 where
x.ltoreq.0.8, y<1, z<1 and y+z<1, where M can be selected
from among Al, Mg and/or Mn; compounds of the formula
n(Li.sub.2MnO.sub.3): n-1(LiNi.sub.1-xM'.sub.xO.sub.2) where M' is
selected from among Co, Mn, Cr and Al and 0<n<1 and
0<x<1.
[0019] In addition, the electrode composition comprises at least
one particulate binder, in particular a polymeric binder. As binder
particles, it is possible to use all particulate polymers which can
be plasticized on at least part of the surface of the binder
particles by heating or by addition of suitable additives, in
particular solvents. This makes formation of an adhesive bond, as
is necessary for the desired agglomeration of the homogeneous
mixture of the particulate components (i) to (iv), on collision
with further particulate components possible.
[0020] As examples of suitable polymers, mention may be made of
thermoplastic polymers, in particular polyolefins (e.g. ethylene-
and/or propylene-containing homopolymers and copolymers),
polyesters (e.g. polyethylene terephthalate (PET)),
polyvinylaromatics (e.g. polystyrene and polystyrene derivatives),
polyacrylates (e.g. polymethyl (meth)acrylate). The conventional
binders known from the field of electrochemical cells, for example
carboxymethyl cellulose (CMC), styrene-butadiene copolymer (SBR),
polyvinylidene fluoride (PVDF), polytetrafluoroethene (PTFE) and
ethylene-propylene-diene terpolymer (EPDM), may also be
emphasized.
[0021] Finally, the electrode composition comprises at least one
particulate pore former. This is characterized in that it serves as
space reserver in the compact electrode during production of the
porous electrode. In addition, the particulate pore former is, in
particular, a component which can remain in the electrochemical
cell and can open possibly closed pores on melting due to its
expansion.
[0022] A compound or a mixture of compounds which can typically
also be used as constituent of an electrolyte composition for an
electrochemical cell is preferably chosen as particulate pore
former. This is associated with the advantage that the pore former
does not have an adverse effect on the properties of the
electrochemical cell. In the case of a suitable choice of the
constituents, it is preferably also possible for the pore former to
be a constituent important for the function of the electrochemical
cell.
[0023] In one embodiment, the particulate pore former is selected
from among at least one lithium salt and/or at least one organic
carbonate which is solid at room temperature. Furthermore, it is
also possible to use additives which can be added to the
electrolyte in order to improve the properties of the latter,
provided that these additives are present as solid. Additives which
serve to bring about the controlled formation of a solid
electrolyte interphase (SEI) may be emphasized. Examples which may
be mentioned are sultones, carbonates and polycyclic
hydrocarbons.
[0024] As preferred lithium salts, mention may be made of those
which are typically used as electrolyte salts in electrolyte
compositions for electrochemical cells. Suitable lithium salts are
preferably selected from the group consisting of lithium halides
(LiCl, LiBr, LiI, LiF), lithium chlorate (LiClO.sub.4), lithium
tetrafluoroborate (LiBF.sub.4), lithium hexafluorophosphate
(LiPF.sub.6), lithium hexafluoroantimonate (LiSbF.sub.6), lithium
hexafluoroarsenate (LiAsF.sub.6), lithium trifluoromethanesulfonate
(LiSO.sub.3CF.sub.3), lithium bis(fluorosulfonyl)imide
(Li[N(SO.sub.2F).sub.2], LiFSI), lithium
bis(trifluoromethylsulfonyl)imide (Li[N(SO.sub.2(CF.sub.3)).sub.2],
LiTFSI), lithium bis(pentafluoroethylsulfonyl)imide
(LiN(SO.sub.2C.sub.2F.sub.5).sub.2), lithium bis(oxalato)borate
(LiB(C.sub.2O.sub.4).sub.2, LiBOB), lithium difluoro(oxalato)borate
(Li[BF.sub.2(C.sub.2O.sub.4)], LiDFOB),
lithium(difluorotri(pentafluoroethyl))phosphate
(LiPF.sub.2(C.sub.2F.sub.5).sub.3) and combinations thereof.
Particular preference is given to lithium salts which have a high
solubility in aprotic, organic, polar solvents.
[0025] As preferred organic carbonates, mention may be made of
those which are present in the solid state at room temperature and
are typically used as constituent of electrolyte compositions for
electrochemical cells. Aprotic, cyclic organic carbonates are
worthy of particular emphasis. These are typically used in
electrolyte compositions in order to improve the high-temperature
stability thereof. In a particularly preferred embodiment of the
invention, the particulate pore former comprises at least one
aprotic, cyclic organic carbonate. Suitable cyclic carbonates are
those having from 3 to 20, preferably from 3 to 10 and in
particular from 3 to 5, carbon atoms. Ethylene carbonate and
propylene carbonate may be emphasized as examples. Particular
preference is given to using ethylene carbonate.
[0026] Suitable further additives which can likewise be used as
pore formers, optionally in mixtures with the abovementioned
compounds, are, in particular, 1,3-propane sultone, vinylene
carbonate (1,3-dioxolen-2-one), vinylethylene carbonate
(4-vinyl-[1,3]-dioxolan-2-one) and fluoroethylene carbonate
(4-fluoro-1,3 -dioxolan-2-one).
[0027] The electrode composition can optionally comprise further
constituents. Conductive additives such as graphite and/or
conductive carbon black, which increase the electrical
conductivity, may be particularly emphasized.
[0028] The amounts of the individual components (i) to (iv) always
have to be matched to the desired composition and porosity and also
the properties of the electrode. The electrode composition usually
comprises a proportion of from 1 to 60% by volume, preferably from
10 to 50% by volume and in particular from 15 to 45% by volume of a
particulate pore former, based on the total volume of the electrode
composition comprising at least the constituents (i), (ii), (iii)
and (iv).
[0029] The constituents of the electrode composition which are also
to be present in the later porous electrode (i.e. the components
(i), (ii) and (iv)) are preferably matched to one another in such a
way that the proportion of at least one binder (ii) is sufficient
to ensure a stable, porous electrode and the proportion of active
material (i) is as high as possible. The electrode composition
preferably comprises from 80 to 99.9% by weight of active material
(i), from 0.1 to 10% by weight of binder (ii) and from 0 to 10% by
weight of conductive additive, based on the total amount of active
material (i), binder (ii) and conductive additive in the electrode
composition. The electrode composition more preferably comprises
from 85 to 95% by weight of active material (i), from 2.5 to 7.5%
by weight of binder (ii) and from 2.5 to 7.5% by weight of
conductive additives (iv), based on the total amount of active
material (i), binder (ii) and conductive additives (iv) in the
electrode composition.
[0030] The electrode composition of the invention is obtained by
intimately mixing the constituents (i), (ii), (iii) and (iv) with
one another. The electrode composition is preferably provided in
the form of a homogeneous, pulverulent composition in process step
(a). This is converted in process step (b) into a shapeable
composition which is obtained by introduction of kinetic and/or
thermal energy. The introduction of energy, in particular of shear
forces, forms fibrils from the binder particles and these fibrils
bring about agglomeration of the electrode composition. This
preferably occurs as a result of the use of mechanical mills such
as jet mills or ball mills.
[0031] In process step (c), the electrode composition is applied as
a layer to the surface of a support material. In one embodiment,
the support material is the surface of a tool, e.g. the surface of
a moving tape. The latter is preferably made of polymer. The layer
can in this case be taken off as free-standing, compact electrode
sheet at the end of the production process. In order to avoid or
reduce adhesion of the compact electrode to the surface of the
support material, the process is preferably carried out at a
temperature below the glass transition temperature T.sub.g of the
at least one binder. The layer can subsequently be detached as
free-standing, compact electrode sheet from the support material
and laminated onto a current collector, e.g. at a temperature above
the glass transition temperature of the binder.
[0032] In a further embodiment, the support material can also be
the surface of a current collector. In this case, no free-standing
electrode sheet is produced but instead an electrode is obtained
straight away.
[0033] The compact electrode can subsequently be compacted by means
of a press, a punch, a roller or a calender. The compaction step
can additionally be carried out under the action of heat in order
to assist adhesion of the binder to the surface of the current
collector and bring about permanent compaction. However, the
temperature is preferably not increased above the melting point of
the at least one pore former.
[0034] In a preferred embodiment of the invention, the process step
(c) comprises a step in which the electrode composition is
compacted. The compact electrode obtained displays a high stability
and low porosity and can thus be processed readily because of the
stability.
[0035] The compact electrode obtained in this way is used in a next
process step (d) in order to produce an electrochemical cell
comprising at least one compact electrode which has been obtained
by the process of the invention and comprises the electrode
composition provided in process step (a). It is also possible to
use a plurality of compact electrodes according to the invention in
an electrochemical cell. The electrochemical cell comprises at
least one negative electrode (anode), at least one positive
electrode (cathode) and at least one separator which is arranged
between the at least one negative electrode and the at least one
positive electrode and separates the two electrodes from one
another.
[0036] The process steps (a), (b), (c) and (d) are preferably
carried out at a temperature at which the particulate pore former
is present as solid. The process temperature during process steps
(a), (b), (c) and (d) is usually below 60.degree. C., more
preferably below 35.degree. C. and in particular below 20.degree.
C.
[0037] In a subsequent, optional process step (e), the compact
electrode which has already been installed in an electrochemical
cell is heated in order to liquefy the at least one particulate
pore former. The compact electrode is in this case preferably
heated to a temperature above the melting point of the at least one
pore former. The heating makes it possible to open up the pores
occupied by the pore former. In addition, possibly inaccessible,
closed pores are opened by expansion of the pore former at elevated
temperatures. The temperature in this process step (e) is
preferably above 20.degree. C., more preferably above 35.degree. C.
and in particular above 60.degree. C. In a particularly preferred
embodiment, the process step (e) is carried out at a temperature
which is at least 5.degree. C. above, in particular at least
10.degree. C. above, the melting point of the at least one
particulate pore former.
[0038] In a further process step (f), the compact electrode is, in
addition to or as an alternative to the process step (e), brought
into contact with at least one liquid electrolyte composition or at
least one liquid constituent of an electrolyte composition for an
electrochemical cell. This is able to dissolve the pore formers
according to the invention. The liquid composition obtained in this
way can remain in the electrochemical cell and in this serves as
electrolyte composition.
[0039] Without being restricted thereto, typical electrolyte
compositions frequently comprise at least one aprotic, organic
solvent as liquid constituent. Examples which may be mentioned are
aprotic nitriles, aprotic ethers, aprotic esters, aprotic
carbonates or a mixture comprising one of the solvents
mentioned.
[0040] Process steps (e) and (f) can also advantageously be carried
out simultaneously. Heating of the compact electrode brings about
opening of any closed pores and additionally increases the
solubility of the preferably molten pore former in the liquid
electrolyte composition. In addition, the time required for filling
the electrochemical cell with the electrolyte composition can be
significantly reduced in this way.
[0041] Process step (f) can also be assisted by the introduction of
the liquid electrolyte composition being carried out under reduced
pressure. Gas inclusions and increased filling times associated
therewith can be avoided in this way.
[0042] In a particularly preferred embodiment of the invention, the
pore former comprises at least one cyclic carbonate, in particular
ethylene carbonate, and the liquid solvent of the electrolyte
composition comprises at least one acyclic carbonate, in particular
dimethyl carbonate.
[0043] Furthermore, the electrolyte composition used preferably
comprises all constituents which cannot be introduced in a
sufficient amount into the electrolyte composition by the
dissolution of the pore formers used. In particular, these are the
abovementioned aprotic, organic cyclic carbonates, the electrolyte
salts mentioned and the additives mentioned.
[0044] The amount of solvent relative to the electrode composition
is preferably selected so that the entire amount of pore formers
can be dissolved therein without adversely affecting the properties
of the electrolyte composition. Rather, the dissolved pore formers
have a positive effect on the properties of the electrolyte
composition, in particular the stability to high temperatures, the
ionic conductivity and/or the formation of the solid electrolyte
interphase.
[0045] As a result of the dissolution of the at least one pore
former present in the (largely nonporous) compact electrode in the
electrolyte composition or the liquid constituent thereof, the
porous electrode and also the actual electrolyte composition of the
electrochemical cell are thus formed in situ.
[0046] The invention also provides a porous electrode for an
electrochemical cell, obtained by the process of the invention.
[0047] The porous electrode of the invention can advantageously be
used as electrode in an electrochemical cell which is preferably
used in a vehicle, in particular in a vehicle having a conventional
internal combustion engine (ICE), in an electric vehicle (EV), in a
hybrid vehicle (HEV) or in a plug-in hybrid vehicle (PHEV).
[0048] The process of the invention makes it possible to produce
electrodes for electrochemical cells without the use of solvents,
with the electrodes having a comparatively high porosity which can
be varied over a wide range. The omission of solvents enables
additional drying steps to be dispensed with. When the electrodes
obtained according to the invention are used in an electrochemical
cell, the last process step of the process for producing the porous
electrode can advantageously be carried out in the finished
electrochemical cell. In this way, the time for filling the pores
of the electrode with the electrolyte composition can be saved
since the pores are formed only in situ with the formation of the
electrolyte composition. A plurality of filling steps is usually
necessary for this purpose.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] Embodiments of the invention will be described in more
detail with the aid of the drawings and the following
description.
[0050] The drawings show:
[0051] FIG. 1 a schematic section of a compact electrode produced
according to the invention before dissolution of the pore formers;
and
[0052] FIG. 2 a schematic section of a porous electrode produced
according to the invention after dissolution of the pore
formers.
DETAILED DESCRIPTION
[0053] FIG. 1 shows a schematic section of a compact electrode 2
produced according to the invention before dissolution of the pore
formers 4. The compact electrode 2 has been applied to a surface of
a substrate 1, in the present case a current collector made of
aluminum. The compact electrode 2 comprises at least one
particulate active material 3 (e.g. an NCM mixed oxide), at least
one particulate pore former 4 (e.g. ethylene carbonate) and also
particulate conductive additives 6 (e.g. conductive carbon black).
The particulate components are joined to one another by fibrils of
binder 5 (e.g. composed of PVDF) and thus form a compact
composition. The compact electrode 2 was produced by homogeneously
mixing particulate active material 3, particulate pore former 4,
particulate binder 5 and conductive additive 6. The binder 5 was
subsequently fibrillated by introduction of shear energy into the
mixture. The fibrils of binder 5 hold the particulate components
together and thus form a shapeable composition which was calendered
onto the surface of a substrate 1.
[0054] FIG. 2 shows a schematic section of the porous electrode 2'
shown in FIG. 1 after the particles of the pore formers 4 have been
dissolved. Pores 7 have been formed at the places concerned.
Dissolution is effected by firstly bringing the porous electrode 2'
to a temperature above the melting point of the pore former 4 and
subsequently bringing the electrode into contact with a solvent at
this temperature. In the present case, a mixture of dimethyl
carbonate and LiPF.sub.6 was used as solvent. The resulting
composition made up of dimethyl carbonate, ethylene carbonate and
LiPF.sub.6 can be used directly as electrolyte composition in the
electrochemical cell in which the compact electrode 2 is used.
[0055] The precise composition of the compact electrode 2 and of
the solvent are described in the following example. The comparative
example describes a conventional electrode composition.
COMPARATIVE EXAMPLE
[0056] A conventional electrode composition usually comprises
90.0 g of active material (NCM mixed oxide) 5.0 g of binder 5.0 g
of conductive additive
[0057] This electrode composition is processed in a jet mill or
ball mill at from 20.degree. C. to 100.degree. C., preferably from
50.degree. C. to 70.degree. C., to give a shapeable composition,
applied to a current collector and compressed. The electrode
obtained is brought into contact under reduced pressure in an
electrochemical cell with an electrolyte composition having the
following composition:
30.92 g of ethylene carbonate 30.92 g of dimethyl carbonate
7.86 g of LiPF.sub.6
[0058] The electrode displays a low porosity.
EXAMPLE
[0059] The electrode composition according to the invention
comprises, for example:
90.0 g of active material (NCM mixed oxide) 5.0 g of binder 5.0 g
of conductive additive 30.92 g of ethylene carbonate
[0060] This electrode composition is processed in a jet mill at
from 20.degree. C. to 50.degree. C. to give a shapeable
composition, applied to a current collector and compressed. The
compact electrode 2' obtained in this way is brought into contact
under reduced pressure in an electrochemical cell with an
electrolyte composition having the following composition:
30.92 g of dimethyl carbonate
7.86 g of LiPF.sub.6
[0061] In contact with the electrode composition under reduced
pressure and at 50.degree. C., the ethylene carbonate is dissolved
out of the compact electrode 2. The properties of the electrolyte
composition are improved in respect of the thermal stability by the
addition of ethylene carbonate. Compared to the conventional
electrode of the comparative example, the porous electrode 2'
displays a high porosity.
[0062] The invention is not restricted to the working examples
described here and the aspects emphasized therein. Rather, many
modifications of the type which a person skilled in the art would
normally make are possible within the scope indicated by the
claims.
* * * * *